backends/drm: don't remove connectors the kernel doesn't consider removed

Removing connectors that are still powered leads to a mismatch in atomic
commits: the crtc is still powered, but the connector also still there.
If KWin tries to disable the crtc afterwards, the atomic commits fail because
the connector needs to be disabled at the same time and it's missing from the
atomic commit request.

To fix this, whenever we fail to fetch information or get wrong data from
the kernel (like 0 modes), use the cached information instead and keep the
connector.

BUG: 456298


(cherry picked from commit a71146c9)